In a conventional BWR (see FIG. 1), the core of nuclear fuel is cooled by water. Feedwater is admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12 and a feedwater sparger 14, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. A core spray inlet 11 supplies water to a core spray sparger 15 via core spray line 13. The feedwater from feedwater sparger 14 flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and core shroud 18. Core shroud 18 is a stainless steel cylinder which surrounds the core 20 comprising numerous fuel assemblies 22 (only two 2.times.2 arrays of which are depicted in FIG. 1). Each fuel assembly is supported at the top by top guide 19 and at the bottom by core plate 21. Water flowing through down-comer annulus 16 then flows to the core lower plenum 24.
The water subsequently enters the fuel assemblies 22 disposed within core 20, wherein a boiling boundary layer (not shown) is established. A mixture of water and steam enters core upper plenum 26 under shroud head 28. Core upper plenum 26 provides standoff between the steam-water mixture exiting core 20 and entering vertical standpipes 30, which are disposed atop shroud head 28 and in fluid communication with core upper plenum 26.
The steam-water mixture flows through standpipes 30 and enters steam separators 32, which are of the axial-flow centrifugal type. The separated liquid water then mixes with feedwater in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus. The steam passes through steam dryers 34 and enters steam dome 36. The steam is withdrawn from the RPV via steam outlet 38.
The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation water outlet 43 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 42 (only one of which is shown) via recirculation water inlets 45. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The pressurized driving water is supplied to each jet pump nozzle 44 via an inlet riser 47, an elbow 48 and an inlet mixer 46 in flow sequence. A typical BWR has 16 to 24 inlet mixers.
The structure of a typical BWR inlet mixer 46 is shown in detail in FIGS. 2A and 2B. In flow sequence starting from the outlet of elbow 48, the inlet mixer comprises: a pre-nozzle section 50; a nozzle section including five nozzles 52 circumferentially distributed at equal angles about the inlet mixer axis; a throat section 54; a barrel section 56; a flare section 58; and a slip joint 60. Each nozzle is tapered at its outlet, so that the nozzle has a maximum diameter d.sub.1 and an exit diameter d.sub.2 which is less than d.sub.1 (see FIG. 2B).
Five secondary inlet openings 62 are circumferentially distributed at equal intervals about the inlet mixer axis. These secondary inlet openings are situated radially outside of the nozzle exits. Therefore, as jets of water exit the nozzles 52, water from the downcomer annulus 16 is drawn into the inlet mixer via the secondary inlet openings, where it is mixed with water from the recirculation pump (not shown).
Experience has shown that during reactor operation, scale forms on critical surfaces of the inlet mixers, including all surfaces from the end of slip joint 60 through 8 inches of the nozzle section. This scale buildup is a significant problem because it causes a loss of cooling flow and reduces reactor output, which is very costly to utilities using nuclear power.
The annular volume between the core shroud 16 and the reactor pressure vessel 10, in which the inlet mixers are located, is difficult to access. Also the complex surfaces and radioactivity of the inlet mixers make mechanical cleaning nearly impossible. At this time, a chemical cleaning method has not been designed for this problem. In addition, the chemicals themselves present disposal problems to the extent that they are not allowed at many reactor sites. Currently, the only method available to eliminate scale buildup is to replace the inlet mixers with new units. However, replacing the inlet mixers is expensive and time consuming for the following reasons: (1 ) building new inlet mixers could take more than one year; (2 ) the reactor would have to be shutdown for a long period of time during installation of the inlet mixers; and (3 ) disposal of the old inlet mixers requires special handling and storage procedures because they are radioactive.